The sulfur reactant container in a sodium sulfur battery is subject to attack in contact with molten sodium polysulfide and sulfur reactants. The prior art has proposed many different materials for combating this corrosion problem.
In U.S. Pat. No. 3,959,013, a proposal is made to use a corrosion resistant and electronically conducting layer of molybdenum or graphite to reduce the corrosion associated with such a battery. The corrosion resistant coating is placed on the surface of the metallic container which is to confine the molten sodium polysulfide and sulfur reactants.
In a similar manner, U.S. Pat. No. 4,048,390 suggests that a protective coating of aluminum be placed on the surface of the battery container which is to confine the polysulfide and sulfur reactants. This patent proposes the use of aluminum because it forms a continuous layer of aluminum sulfide over its exposed surfaces.
U.S. Pat. No. 4,110,516 takes still another direction in trying to develop a corrosion resistant container to confine the sodium polysulfide and sulfur reactants. This patent suggests forming the confining container of aluminum and then placing over the aluminum either a single layer of chrome or a layer of zinc with a layer of chrome thereover, the chrome surface being the surface which faces up against the corrosive reactants which are to be confined therewithin.
U.S. Pat. No. 4,131,226 once again discloses a sulfur containing container for a sodium sulfur battery in which a liner material is used as an anticorrosive surface for a mild steel container. The patent teaches that the discreet liner of clad material can be formed of metal such as stainless steel, molybdenum or a nickel/chromium alloy, as specifically disclosed therein.
The above patents constitute the best prior art known to us in this area. No search was conducted in the U.S. Patent Office on this subject matter.
We propose that the protective coating for the container which is to confine the molten sodium polysulfide and sulfur reactants be protected by a chromium/chromium oxide coating. If the metal container is formed of a material such as mild steel, and it is protected from corrosion by the polysulfide materials with a noncorroding coating of reasonable conductivity, then the metal of the container provides the high axial conductivity required for current collection in a cylindrical cell while it provides the corrosion protection.
Chrome oxide is fairly noncorrodible in polysulfide melts. It does not, however, have a sufficient conductivity. But, when alloyed with chromium, the resultant chromium/chromium oxide combination does have the required conductivity while retaining the noncorrodibility of the chromium oxide. Under attack by the polysulfide melts, the chromium in the alloy corrodes to form a chromium sulfide corrosion product which fills the pores in the alloy sealing the same off. The chromium sulfide is a good conductor, and it also protects the underlying chromium from further corrosion. The chromium oxide matrix serves as an anchor for retention of the protective chromium sulfide layer.
It is well known to those skilled in the art of constructing sodium sulfur cells that chromium and stainless steels are protected from further corrosion by a chromium sulfide corrosion scale. However, when the chromium is there by itself, this scale has a tendency to spall or flake off unless it is retained within a porous matrix by a noncorrodible material.
The principal objects of this invention are to provide an improved protection system for a sodium sulfur battery wherein the metal current collector and/or container is coated with an alloy or cermet which is nearly as conductive as a metal, nearly as inert to corrosion as an oxide, and a coating which protects the substrate from defects and porosity in the coating by means of a protective, nonsluffing corrosion scale.